Highly transparent impact-resistant glass ceramic

ABSTRACT

The present invention describes a transparent plate of lithium aluminosilicate glass ceramic showing a high transmission, a process for producing same and transparent plate laminates comprising at least one plate of the lithium aluminosilicate glass ceramic of the invention and the use thereof as armored glass or bullet-proof vest.

The present invention describes a transparent plate made of lithiumaluminosilicate glass ceramic showing high transmission, a process formaking same and transparent plate laminates comprising at least oneplate of the lithium aluminosilicate glass ceramic according to theinvention and the use thereof as armored glass or bullet-proof vest.

Until now, the inherent color of transparent glass ceramics has been toostrong. The reasons for the inherent color of transparent glass ceramicscan vary. The constituents of the raw material mixtures for the meltscontain the coloring element Fe as an impurity. The use of the refiningagents Sb₂O₃ and CeO₂ also results in a slight inherent color. Thedescribed brownish-yellow intrinsic color of the transparent glassceramics is based substantially on electronic transitions occurring oncolored complexes which absorb in the region of visible light and inwhich the component required for nucleation, namely the Ti ion, takespart. The most frequent absorbing color complex stems from the formationof adjacent Fe and Ti ions between which electronic charge transferstake place. Sn—Ti complexes also impart an inherent color. The Fe/Ticolor complexes lead to a red-brown discoloration and the Sn/Ti colorcomplexes to a yellow-brown one. The formation of these adjacent colorcomplexes takes place already during the cooling of the parent glass andparticularly during the subsequent ceramization of the glass ceramic. Inthe melt, the ions are still uniformly distributed, but during coolingat high temperatures and during ceramization they preferably bind toeach other. As a result, during the ceramization of the transparentglass ceramics, the inherent color intensifies very markedly compared tothat of the parent glass. By absorption in the short-wave region of thevisible spectrum, transparent flat glasses and particularly the glassceramics produced therefrom assume a pronounced inherent color whichincreases considerably with thickness.

It is known that the inherent color of glass ceramics can be reduced byovercoloring. The principle of overcoloring an undesirable color tingenaturally leads to stronger light absorption thus reducing the overalltransmission, because the absorptions taking place are neutralized bythe absorptions of complementary light portions by the overcoloringagent.

Glass ceramic plates find use in, among other applications, bullet-proofglass plates. In the production of such glass plates, several differentglass or glass ceramic layers and plastic sheets are linked. Thetemperature- and pressure-controlled process of linking the individuallayers and plastic materials to each other, in particular, istime-consuming and cost-intensive. The many interfacial transitionsbetween glass plates and plastic materials result in poor transmissioncharacteristics which may lead to the formation of interference fringepatterns in the form of Newton fringes. Also, the large amount of glass,namely the high number of glass plates in the known bullet-proof platesresults in their exhibiting a very high weight per unit area. The highweight per unit area leads to a pronounced constructive cost ofinstallation and vitrification.

Hence, one object of the present invention is to provide plates of glassceramic free of the disadvantages shown by the prior-art plates andwhich are suitable for producing armored glass.

Another object of the present invention is to provide glass ceramicplates exhibiting high overall transmission for visible light. Inparticular, these glass ceramic plates show a high overall trans-missionfor visible light which manifests itself in a high brightness value fortransmitted standard light A under 2° observation (Y_(A/2)).

Another object of the present invention is to provide plate laminates,comprising at least one glass ceramic plate, exhibiting high overalltransmission for visible light and/or a high hardness. In particular,these plate laminates show a high overall transmission for visible lightand their light brightness value Y for transmitted standard light Aunder 2° observation is Y_(A/2°)>50.

Another object of the present invention is to provide plate laminates,comprising at least one glass ceramic plate, said laminates exhibitingbetter resistance to dynamic stresses compared to plate laminates withconventional glass ceramic plates and the same weight per unit area.

Another object of the present invention is to provide plate laminates,comprising at least one glass ceramic plate, said laminates exhibitingimproved transparency thus ensuring bullet impact resistance meeting therequirements of NATO Standardization Agreement, STANAG 4569, Level 2 and3. The plate laminates also ensure bullet impact resistance againstarmor-piercing projectiles, namely against armor-breaking projectiles.

The afore-indicated objectives are reached by providing a transparentplate made of lithium aluminosilicate glass ceramic containing thefollowing constituents in weight %, based on the overall composition:

Li₂O 3.0-4.5 Al₂O₃ 18.0-24.0 SiO₂ 55.0-70.0 TiO₂   0-2.3 SnO₂   0-0.2ZrO₂   0-2.0 ΣTiO₂ + ZrO₂ 0.5-4.3 MgO   0-0.8 Fe2O₃ 40-200 ppm and As₂O₃0.3-0.9 wt. % as chemical refining agent.

The plate preferably contains from 40 to 130 ppm of Fe₂O₃, and the TiO₂content is preferably higher than 0.01 wt. % and particularly higherthan 0.5 wt. %.

The reduction in Fe content is economically feasible only to a certaindegree. A certain amount of Fe or Fe₂O₃ always enters the mixture withthe industrially used raw materials for the production of the glass andwith the abraded material from the units for the production,homogenization and transportation of the mixture. Because of theelevated cost of high-purity raw materials or of the special measuresapplied to industrial units, it is economically no longer feasible todrop the Fe₂O₃ content of industrially produced transparent glassceramics below about 40 ppm.

For purposes of the present invention, by a glass ceramic is meant aninorganic, nonporous material with a crystalline phase and a glass phasein which, as a rule, the matrix, namely the continuous phase, is a glassphase. The combination of a crystalline and a glass phase imparts to aglass ceramic its special properties.

For purposes of the present invention, visible light is light with awavelength from 380 to 780 nm.

The brightness value Y of the CIE-xyY color-measuring system is alwaysreported for transmitted standard light A and for an observation angleof 2° and can be determined from wavelength-resolved transmissionspectra with the aid of the CIE-defined eye-sensitivity curves x(λ),y(λ) and z(λ) (tristimulus curves) (International IlluminationCommission Proceedings, 1931, Cambridge University Press, Cambridge, orDIN 5031):

$X = {\int_{0}^{\infty}{{I(\lambda)}{\overset{\_}{x}(\lambda)}\ {\lambda}}}$$Y = {\int_{0}^{\infty}{{I(\lambda)}{\overset{\_}{y}(\lambda)}\ {(\lambda)}}}$$Z = {\int_{0}^{\infty}{{I(\lambda)}{\overset{\_}{z}(\lambda)}\ {\lambda}}}$

λ: wavelength of monochromatic light;I(λ): intensity of monochromatic light [I(λ)=τ(λ)×f_(A/2°)(λ);wavelength-solved transmission of the sample τ(λ) multiplied by thewavelength-solved intensity factor for standard light A observed under a2° angle f_(A/2°)].

The factor coordinates can be calculated from:

$x = \frac{X}{X + Y + Z}$ $y = \frac{Y}{X + Y + Z}$$z = \frac{Z}{X + Y + Z}$

The achromatic point for standard light A is defined by:

x_(u,A)=0.4476

Y_(u,A)=0.4074

The abbreviation CTE refers to the linear thermal expansion coefficientof a material that can be indicated for different temperature ranges.CTE (ΔI/I₀ΔT) in ppm/K is determined by dilatometric measurements with aThermal Dilatometric Analyzer, Harrop model TD 710, in accordance withDIN ISO 7991.

Preferably, the transparent plate contains the following additionalconstituents, in wt. % based on the overall composition:

BaO 0-3   Na₂O 0-1.5 ZnO 0-2.5 ZrO₂ 0.5-2   

The oxides Li₂O, Al₂O₃ and SiO₂ are required constituents of the glassceramic and are present within the preferred limits cited in the claims.In general, a minimum amount of 3 wt. % of Li₂O is needed, but Li₂Ocontents exceeding 4.5 wt. % often cause undesirable devitrificationduring the manufacturing process. An Li₂O content of 3.2 to 4.3 wt. %gives particularly good results.

To prevent excessively high glass viscosities and to suppress thetendency toward undesirable devitrification during shaping, the Al₂O₃content is limited to a preferred minimum of 18 wt. % to a preferredmaximum of 24 wt. %. When the Al₂O₃ content is below 19 wt. %, thetransparency of the glass ceramic is reduced. The SiO₂ content shouldpreferably amount to a maximum of 70 wt. %, particularly 68 wt. %,because this component causes a marked increase in the viscosity of theglass. Hence, higher SiO₂ contents are deleterious for the melting ofthe glasses because of the temperature stress exerted during shaping.The minimum SiO₂ content should preferably amount to 55 wt. % andparticularly 60 wt. %.

MgO and ZnO can be incorporated into the crystalline phase as additionalcomponents. Because of the problem of undesirable formation of crystalphases such as Zn spinel during ceramization, the ZnO content is limitedto a maximum of 2.5 wt. % and preferably to a maximum of 2 wt. %. TheMgO content is limited to a maximum of 0.8 wt. % and preferably to amaximum of 0.65 wt. %, because otherwise the expansion coefficient ofthe glass ceramic is increased inadmissibly. A low MgO content is alsoadvantageous in suppressing the inherent color of the glass ceramic. Asa rule, a minimum MgO content of 0.3 wt. % is required to prevent thethermal expansion of the glass ceramic in the temperature range from 30°C. to 700° C. from decreasing to negative values below −0.3×10⁶/K

The glass ceramic contains as nucleating agents TiO₂ and ZrO₂ andpreferably only TiO₂. SnO₂ can also serve as nucleating agent. Theamount of TiO₂ used is preferably in the range between 1.8 and 2.3 wt. %and most preferably between 2.0 and 2.3 wt. %. The amount of ZrO₂ ispreferably in the range between 0.5 and 2 wt. % and most preferablybetween 1.5 and 2 wt. %. If the two nucleating agents are used at thesame time, the sum of TiO₂+ZrO₂ should not exceed 4.3 wt. % andpreferably not 2.3 wt. %.

The afore-indicated amounts of nucleating agents allow ceramization tobe carried out in a short time, preferably within a period of 1 to 2hours. Because of the low amount of TiO₂, the strength of the inherentcolor is also reduced.

ZnO, MgO and BaO, are added to the composition to improve the meltproperties of the glass ceramic and to stabilize the glass phase. ZnOalso makes it possible to influence the thermal expansion coefficient(CTE) while MgO and BaO cause this coefficient to increase. Also, highercontents of the afore-said metals can affect the crystallizationbehavior during the conversion of the glass into the glass ceramic andthey exert a deleterious effect on the thermal stress resistance of theglass ceramic. Preferably, the glass ceramic of the present inventioncontains 1 to 2.6 wt. % of ZnO, 0.3 to 0.8 wt. % of MgO and 0 to 3 wt. %of BaO. Most preferably, the glass ceramic of the present inventioncontains 1 to 2 ZnO, 0.3 to 0.65 MgO and 0 to 2 BaO, each expressed inwt. %.

The glass ceramic of the present invention can also contain 0 to 1.5 wt.% and preferably 0 to 1 wt. % of Na₂O. After ceramization, the alkalimetal ions, for example the sodium ions, remain in the residual glassphase. They increase the thermal expansion coefficient and can thereforebe used when the thermal expansion coefficient values are too negative.If, however, they are used in excessive amounts, the thermal expansionbecomes too high, and the nucleation is difficult to control.

The glass ceramics according to the invention are refined by use ofarsenic oxide, the refining agent commonly used in the LiO₂—Al₂O₃—SiO₂system. Alternatively or in combination, SnO₂ can also be used inamounts of up to 0.2 wt. %, especially for high-temperature refiningat >1700° C. Other refining agents, for example, Sb₂O₃, CeO₂, sulfatecompounds, chloride compounds or fluoride compounds, can also be addedto the glass melt. The total amount of refining agents and additivesshould not exceed 1.2 wt. %. Preferably, As₂O₃ is used as the onlyrefining agent.

At low contents of As₂O₃, Sb₂O₃ or SnO₂ refining agents, it may benecessary to combine the chemical refining with high-temperaturerefining above 1700° C. if good bubble quality with bubble numbers below5 bubbles/kg of glass (based on bubble sizes >0.1 mm) is desired. It isparticularly advantageous, if inherent color is to be avoided, to refinethe glass ceramic exclusively with As₂O₃ as the refining agent and notto use antimony oxide and tin oxide as refining agents. Other refiningagents, such as sulfate, chloride or fluoride compounds, may also beadded in a total amount of up to 1 wt. %.

Preferably, the thickness of the transparent glass ceramic plate is inthe range from 2 and 20 mm, particularly in the range from 4 and 15 mmand especially in the range from 6 and 12 mm.

The transparent plate according to the invention, at a thickness of 4mm, preferably shows a brightness value Y_(A/2°) of ≧80, preferably ≧85and most preferably ≧89.

The thermal expansion coefficient (CTE) between 30 and 700° C. ispreferably in the range between −0.15 to +0.15×10⁻⁶/K and morepreferably in the range between −0.05 to 0.1×10⁻⁶/K.

The Knoop hardness of a glass ceramic according to the invention isdetermined in accordance with DIN ISO 9385, edition of 1991-01. TheKnoop diamond is impressed under a load of 0.1 N for 20 seconds. TheKnoop hardness of the glass ceramic plate of the invention isHK_(0.1/20)≧500, preferably HK_(0.1/20)≧550 and most preferablyHK_(0.1/20)≧580.

Another object of the invention is a process for producing the glassceramic of the invention.

It is generally known that glasses of the Li₂O—Al₂O₃—SiO₂ system can beconverted into glass ceramics in which high-quartz mixed crystals arethe main crystal phase. To this end, the following procedure is used: Asuitable composition of suitable raw materials is melted, refined,homogenized and then, while hot, shaped to form a glass blank or greenbody, for example by rolling, casting, pressing or, recently, floating.By “green body” of a glass ceramic is meant a glassy body obtained froma melt of suitable composition which by treatment according to asuitable temperature program can be converted into a glass ceramic.

The cooling and annealing of the molten green body is followed by a heattreatment whereby the glass is converted into a glass ceramic bycontrolled bulk crystallization. In the course of this heat treatment,in a first conversion step (“nucleation agent separation”)crystallization nuclei of the same or different kind are formed in theglass. By crystallization nuclei or crystal nuclei are meantsubmicroscopic crystalline aggregates of a characteristic size. In asecond conversion step (“crystallization”), possibly at a slightlyhigher temperature, crystals or crystallites grow on the crystal nuclei.The glass ceramic of the invention is preferably produced in accordancewith the following ceramization program:

-   -   heating to a temperature of 750° C./±20° C. and holding this        temperature for 20 minutes±15 minutes;    -   further heating, for purposes of ceramization, to a temperature        of 900° C.±20° C. and holding this temperature for 20 minutes±15        minutes, then cooling to room temperature.

The cooling to room temperature requires no particular temperatureprogram. As a rule, it is carried out by exposing the hot plates to roomair. The cooling time usually ranges from 10 K/min to <−100 K/min [sic].

Another object of the present invention is a transparent plate oflithium aluminosilicate glass ceramic which contains the followingconstituents, expressed in wt. % based on the total composition:

Li₂O 3.0-4.5 Al₂O₃ 18.0-24.0 SiO₂ 55.0-70.0 TiO₂   0-2.3 SnO₂   0-0.2ZrO₂   0-2.0 ΣZrO₂ + TiO₂ 0.5-4.3 MgO   0-0.8 Fe₂O₃ 40-200 ppm and As₂O₃0.3-0.9 wt. % as chemical refining agent.and is prepared in accordance with the following ceramization program:

-   -   heating for the purpose of nucleation to a temperature of 750°        C.±20° C. and holding this temperature for 20 minutes±15        minutes;    -   further heating for the purpose of ceramization to a temperature        of 900° C.±20° C. and holding this temperature for 20 minutes±15        minutes, then cooling to room temperature.

The afore-described transparent glass ceramic plate is appropriate forthe production of plate laminates with a high bullet penetrationresistance and high transparency for visible and infrared light in thewavelength range between 380 and 1100 nm. Moreover, the thermomechanicalproperties of the glass ceramic according to the invention make itpossible to use it in fire safety glazing, as fireplace sight glass,ceramic hob, substrate for semiconductor materials or substrate formagnetic storage plates.

Another object of the present invention is a transparent plate laminatewhich, in particular, shows a high resistance to bullet penetration anda high optical transmission.

The plate laminate according to the invention comprises at least onetransparent glass ceramic plate (a), as described in the foregoing,optionally at least one plate (b) selected from the group consisting ofborosilicate glass, soda lime glass and aluminosilicate glass which canalso be chemically or thermally prestressed, and at least one plate (c)selected from the group consisting of polycarbonate, polyacrylate,particularly poly(methyl methacrylate), cellulose acetate butyrate,nylon, polyolefin, polyester, polyurethane and a mixture thereof.

According to German industrial standard DIN 52290, part 3 (06/1984),and/or according to German industrial standard DIN 52290, part 2(11/1988), the plate laminate of the invention exertsbreakthrough-inhibiting properties. The plate laminate according to theinvention also ensures bullet penetration resistance that meets therequirements of the NATO Standardization Agreement, STANAG 4569, Levels2 and 3.

Preferably, plate (c) consists of transparent polycarbonate (PC),poly(methyl methacrylate) (PMMA) or polyurethane (PU).

The number of plates in the transparent plate laminate is limited onlyby the requirements for high transmission it must meet. The number ofplates (a), (b) and (c) preferably amounts to 2 to 10, more preferablyto 3 to 9 and most preferably to 4 to 8. A plate (a) is preferablyoriented outward or is furthest away from the object to be protected.For armored glass, for example, this means that plate (a) is the firstplate that must withstand external influences. Preferably, plate (c) isoriented closest to the object to be protected. In the case of armoredglass, this means that plate (c) is the plate that lines the internalspace of a motor vehicle.

Preferred is a plate arrangement that shows the following platesequence, in all cases from the outside (furthest away from the objectto be protected) toward the inside (closest to the object to beprotected):

8-plate laminate(a)-(a)-(a)-(a)-(a)-(a)-(a)-(c)(b)-(b)-(a)-(a)-(b)-(b)-(a)-(c)(a)-(a)-(a)-(b)-(b)-(b)-(b)-(c)(a)-(a)-(b)-(b)-(a)-(a)-(a)-(c)(a)-(a)-(b)-(b)-(a)-(a)-(b)-(c)7-plate laminate(a)-(a)-(a)-(b)-(b)-(b)-(c)(b)-(b)-(b)-(a)-(a)-(b)-(c)(a)-(a)-(b)-(a)-(b)-(b)-(c)5-plate laminate [sic](a)-(a)-(b)-(b)-(a)-(c)(a)-(a)-(a)-(b)-(b)-(c)(a)-(b)-(a)-(b)-(b)-(c)

Plate (b) preferably has a thickness ranging from 3 to 20 mm,particularly from 5 to 15 mm and most preferably from 5 to 10 mm. Thethickness of plate (c) is preferably in the range from 3 to 15 mm,particularly from 5 to 15 mm and most preferably from 8 to 13 mm. Thethickness of plate (a) in the plate laminate is preferably in the rangefrom 3 to 20 mm, particularly in the range from 4 to 15 mm and mostpreferably in the range from 6 to 12 mm.

The thickness of the plate laminate is preferably in the range from 30to 100 mm, particularly in the range from 40 to 80 mm and mostpreferably in the range from 60 to 80 mm.

Preferably, plates (a)-(a), (a)-(b) and (b)-(b), independently of eachother, are attached to each other with a bonding agent selected from thegroup consisting of casting resins or reactive resins based onpolyurethanes, polyvinylbutyral (PVB), cross-linked polyurethanes,partly cross-linked polyurethanes, polyureas, epoxides, unsaturated orsaturated polyesters, polybutylene terephthalates (PBT),poly(meth)acrylates, silicones or silicone resin polymers, or from thegroup of hot-melt adhesives, coatings or sealants selected from thegroup consisting of hot-melt adhesives based on polyethylene orcopolymers thereof, particularly ethylene vinylacetate (EVA) orpolyvinyl acetate or mixtures thereof. More preferably plates (a)-(a),(a)-(b) and (b)-(b), independently of each other, are connected to eachother with a bonding agent selected from the group consisting of castingresins or reactive resins based on polyurethanes, polyvinylbutyral andhot-melt adhesives based on ethylene vinylacetate. Most preferably,plates (a)-(a), (a)-(b) and (b)-(b) are connected with polyvinylbutyralor polyurethane in the form of a film.

Plates (a)-(c), (b)-(c) and (c)-(c), independently of each other, areattached to each other with a bonding agent selected from the groupconsisting of casting resins or reactive resins based on polyurethanes,polyvinylbutyral (PVB), cross-linked polyurethanes, partly cross-linkedpolyurethanes, polyureas, epoxides, unsaturated or saturated polyesters,polyethylene terephthalates (PET), polybutylene terephthalates (PBT),poly(meth)acrylates, silicones and silicone resin polymers, or from thegroup of hot-melt adhesives, coatings or sealants selected from thegroup consisting of hot-melt adhesives based on polyethylene orcopolymers thereof, particularly ethylene vinylacetate (EVA), orpolyvinyl acetate or mixture thereof. Most preferably, the (a)-(c),(b)-(c) and (c)-(c) plates, independently of each other, are connectedby means of a bonding agent selected from the group consisting ofcasting resins or reactive resins based on polyurethanes,polyvinylbutyral and hot-melt adhesives based on ethylene vinylacetate.Most preferably, the (a)-(c), (b)-(c) and (c)-(c) plates are connectedby means of polyvinylbutyral in the form of a film.

A transparent plate laminate according to the present invention thatuses the transparent glass ceramic plates of the invention shows a lowerweight per unit area than do corresponding trans-parent plate laminatescontaining conventional glasses or glass ceramics. Preferably, theweight per unit area of the plate laminate is in the range between 50and 150 kg/m², more preferably in the range between 50 and 120 kg/m² andmost preferably in the range between 50 and 100 kg/m² and it meets therequirement of “NATO AEP-55 STANAG 4569—Level II or III” by stopping,for example, hard-core bullets of the 7.62×39 API BZ type striking at695 m/s, and bullets with a tungsten carbide core of the 7.62×51 AP typestriking at 930 m/s or hard-core bullets of the 7.62×54R B32 API typestriking at 854 m/s.

The thickness of the bonding agent layers preferably ranges from 0.05 to2 mm, more preferably from 0.1 to 1 mm and most preferably from 0.2 to0.8 mm.

Another object of the present invention is a process for producing theplate laminate that comprises the following steps:

-   -   cutting out plates (a), (b) and (c),    -   cleaning plates (a), (b) and (c),    -   stacking up the plates and introducing a bonding agent into the        spaces between the individual plates (a), (b) and (c),    -   possibly exposing the stack to heat treatment to activate the        bonding agent, possibly under vacuum or by exerting high        pressure on the stack,    -   cooling the laminate.

A similar process for producing a laminate is described, for example, inUS 2008-187721 AA (SO-CLIMA GmbH) or EP 0 331 648.

By the invention-provided possibility to use only few and possibly thinglass ceramic plates, the transmission properties of the plate laminateof the invention are much improved. Also, the plate laminates accordingto the invention possess outstanding color neutrality because of the useof glass ceramic plates which can be thinner compared to those of theprior art.

Another object of the present invention is the use of a glass ceramicplate according to the invention or the use of a plate laminateaccording to the invention, as described in the foregoing, as part of anarmored glass or as part of a bullet-proof vest.

The armored glass according to the invention ensures maximum energyabsorption when a bullet strikes. This splinter-proof design preferablyserves to protect the persons present in a motor vehicle or buildingprovided with such an armored glass from being hit by flying glasssplinters stemming from a glass pane hit by a bullet.

DESCRIPTION OF THE FIGURES

FIG. 1 Description of a plate laminate of the invention

G1 to G8 represent plates made of glass, glass ceramic or plasticmaterial.

K1 to K7 represent layers of bonding agents based on a plastic material,particularly films of polyvinylbutyral or polyurethane that as a ruleare currently used for the production of laminated glass.

EXAMPLES

Four different glass ceramics of different composition were prepared(data are given in wt. % based on oxides). The brightness value Y_(A/2°)for a glass thickness of 4 mm and the linear thermal expansioncoefficient (CTE) are given for the range between 30 and 70° C.

To prepare the glass ceramics, first a green glass of the samecomposition was prepared in the usual manner. From this green glass,glass plates of the desired thickness were then made. The glass plateswere then ceramized in the known manner to form glass ceramic plates,the nucleating agents having been formed in the glass at a temperatureof 750° C. and a residence time of 20 minutes. The glass was then heatedto the crystallization temperature of 830° C. The crystallization wasallowed to occur at a temperature ranging from 830° C. to 900° C., thetemperature being raised from 830° C. to 900° C. at a rate of 10 K perminute with a 10-minute residence time at 900° C.

To determine the brightness value Y_(A/2°), first thewavelength-dependent transmission τ(λ) of a 4-mm-thick sample of theglass ceramic was measured and standardized to normal light A

I(λ)=τ(λ)×f _(A)(λ) (λ=350 . . . 800 nm)

Then Y was then calculated with the aid of the CIE-defined eyesensitivity curves y_(avg)(λ) (International Illumination CommissionProceedings, 1931, Cambridge University Press, Cambridge, or DIN 5031):

$Y = {\int_{\lambda = {380\mspace{14mu} n\; m}}^{\lambda = {800\mspace{14mu} n\; m}}{{I(\lambda)}{\overset{\_}{y}(\lambda)}\ {\lambda}}}$

The linear thermal expansion coefficient CTE was determined in the 30 .. . 700° C. temperature range in accordance with DIN ISO 7991 by use ofa Thermal Dilatometric Analyzer, Harrop model TD 710.

The compositions, brightness values and expansion coefficients arecollected in Table 1.

1 2 3 4 Al₂O₃ 21.96 21.70 21.50 21.67 As₂O₃ 0.35 0.31 0.31 0.85 BaO 2.001.99 1.94 1.97 Fe₂O₃ 0.0050 0.0110 0.0120 0.0130 Li₂O 3.78 3.76 3.663.72 MgO 0.60 0.58 0.58 0.58 Na₂O 0.55 0.53 0.52 0.52 SiO₂ 65.04 65.5065.80 64.96 TiO₂ 2.27 2.11 2.29 2.29 ZnO 1.70 1.68 1.64 1.67 ZrO₂ 1.731.80 1.76 1.77 SnO₂ <0.01 <0.01 <0.01 <0.01 Y(A/2°); 4 mm 89.7 89.5 89.389.7 CTE (30 . . . 700° C.) −0.08 −0.11 −0.1 −0.1

The glass ceramic of the invention according to Example 4 wasincorporated into a plate laminate. The plate laminate was prepared inan autoclave by common lamination methods using different glass andglass ceramic plates by interposing in each case one PVB film inaccordance with the process parameters recommended by the manufacturerof the PVB film (Butacite® Clear, manufactured by DuPont).

Eleven plate laminates of different structures were prepared and aresummarized in Table 2.

In Table 2, G1 to G8 indicate the bullet-proof layers and K1 to K7 thelayers of the lamination film. The constituents of the individual layersare numbered, and when necessary their thickness is indicated in mm. Thenumbering of the individual layers has the following meaning:

-   -   1: Glass ceramic of the invention as per Example 4, Table 1.    -   2: Floated borosilicate glass with an expansion coefficient        CTE_(30 . . . 300) of 3.3×10⁻⁶K⁻¹ (Borofloat® 33, Schott AG,        composition in wt. % about 81 SiO₂, 13 B₂O₃, 4 Na₂O+K₂O, 2        Al₂O₃).    -   5: Prior-art glass ceramic (EP 1,837,312, Example 3, rounded off        composition in wt. %: 65.3 SiO₂, 21.8 Al₂O₃, 3.7 Li₂O, 2.3 TiO₂,        2 BaO, 1.7 ZnO, 1.8 ZrO₂, 0.6 MgO, 0.5 Na₂O, 0.3 As₂O₃, 0.1        Nd₂O₃).    -   6: Commercial impact-resistant poly(methyl methacrylate)        (Plexi-glas Resist®, Evonic Industries, U.S. Pat. No. 5,726,245        A).    -   7: Polyvinylbutyrate film, thickness 0.38 mm (Butacite® Clear,        DuPont).

To determine the weight per unit area, kg×m⁻², a 50×50 cm² plate wasweighed and the result was converted into 1×m². The antiballistic limitm×s⁻¹ was determined with a 7.62-mm caliber bullet weighing 10 g andcontaining steel core. Different bullet speeds were produced by means ofdifferent propelling charges. To determine the antiballistic limit, thetest laminates were shot at with projectiles of different speed, and theantiballistic limit was then determined on the basis of the impactpattern for the different speeds.

Table 2 clearly indicates that plate laminates of the glass ceramic ofthe invention show a lower weight per unit area than do plate laminatesof a different composition but with a comparable ballistic limit.

TABLE 2 Wt./ unity Anti- Thick- area, ballis- Exam- ness kg/ (A/ tic pleG1 K1 G2 K2 G3 K3 G4 K4 G5 K5 G6 K6 G7 K7 G8 mm, m² 2°) limit 6b 2 7 2 72 7 2 7 2 7 2 7 2 7 6 78.66 158 52.35 820 4 mm 10 mm  10 mm  10 mm  10mm  10 mm  10 mm  12 mm 6a 2 7 2 7 1 7 1 7 2 7 1 7 1 7 6 70.66 151.551.2 830 4 mm 10 mm  8 mm 8 mm 10 mm  8 mm 8 mm 12 mm 6 2 7 1 7 1 7 1 71 7 1 7 1 7 6 66.66 144 50.72 850 4 mm 8 mm 8 mm 8 mm 8 mm 8 mm 8 mm 12mm 5 1 7 1 7 1 7 1 7 1 7 1 7 1 7 6 65.66 147 49.84 860 3 mm 8 mm 8 mm 8mm 8 mm 8 mm 8 mm 12 mm 4a 1 7 1 7 1 7 2 7 2 7 2 7 6 7 63.28 131 53.9760 3 mm 8 mm 8 mm 10 mm  10 mm  10 mm  12 mm  4 1 7 1 7 1 7 1 7 1 7 1 76 57.28 126 53.11 780 3 mm 8 mm 8 mm 8 mm 8 mm 8 mm 12 mm  3a 1 7 1 7 17 2 7 2 7 6 7 52.9 109 57.17 620 3 mm 8 mm 8 mm 10 mm  10 mm  12 mm  3 17 1 7 1 7 1 7 1 7 6 48.9 105.5 56.6 630 3 mm 8 mm 8 mm 8 mm 8 mm 12 mm 2a 1 7 1 7 1 7 2 7 6 44.52 88 60.96 520 3 mm 8 mm 8 mm 10 mm  12 mm  2 17 1 7 2 7 1 7 6 40.52 85 60.33 520 3 mm 8 mm 10 mm 8 mm 12 mm  1 5 7 5 75 7 5 7 6 45.52 97.5 49.4 600 8 mm 8 mm 8 mm 8 mm 12 mm 

1. Transparent plate of lithium aluminosilicate glass ceramic having acomposition containing, expressed in wt. % based on the totalcomposition Li₂O 3.0-4.5 Al₂O₃ 18.0-24.0 SiO₂ 55.0-70.0 TiO₂ 0.01-2.3 ZrO₂ 0.01-2.0  ΣTiO₂ + ZrO₂ 0.5-4.3 SnO₂   0-0.2 MgO   0-0.8 Fe2O₃40-200 ppm and As₂O₃ 0.3-0.9 wt % as chemical refining agent.


2. Transparent plate according to claim 1, characterized in that theplate contains from 40 to 130 ppm of Fe₂O₃.
 3. Transparent plateaccording to claim 1 or 2, characterized in that the compositioncontains as additional constituents BaO 0-3   Na₂O 0-1.5 ZnO 0-2.5

expressed in wt. %, based on the total composition.
 4. Transparent plateaccording to one or more of claims 1 to 3, characterized in that thethickness of the plate is in the range from 3 to 20 mm, preferably inthe range from 4 to 15 mm and more preferably in the range from 6 to 12mm.
 5. Transparent plate according to one or more of claims 1 to 4,characterized in that the brightness value for transmitted normal lightobserved at an angle of 2°, Y_(A/2°), for a plate thickness of 4 mm is≧80, preferably ≧85 and more preferably ≧89.
 6. Transparent plateaccording to one or more of claims 1 to 5, characterized by the factthat between 30 and 700° C. the thermal expansion coefficient (CTE)ranges from −0.15 to +0.15×10⁻⁶/K, preferably from −0.05 to −0.12×10⁻⁶/Kand more preferably from −0.05 to −0.1×10⁻⁶/K.
 7. Process for producinga transparent plate according to one or more of claims 1 to 6 comprisingthe following steps: preparing a green body and using a ceramizationprogram involving heating, for the purpose of nucleation, to atemperature of 750° C.±20° C. and holding this temperature for 20minutes±15 minutes, further heating, for the purpose of ceramization, toa temperature of 900° C.±20° C. and holding this temperature for 20minutes±15 minutes and cooling to room temperature.
 8. Transparent platelaminate comprising at least one transparent plate (a) according to oneor more of claims 1 to 6, possibly at least one plate (b) selected fromthe group consisting of borosilicate glass, soda lime glass,aluminosilicate glass or chemically or thermally prestressedborosilicate glass, aluminosilicate glass or soda lime glass and atleast one plate (c) selected from the group consisting of polycarbonate,polyacrylate, particularly poly(methyl methacrylate), cellulose acetatebutyrate, nylon, polyolefin, polyester, polyurethane and mixturesthereof.
 9. Transparent plate laminate according to claim 8,characterized in that the number of plates (a), (b) and (c) is 20 to 10,preferably 3 to 9 and most preferably 4 to
 8. 10. Transparent platelaminate according to claim 8 or 9, characterized in that the thicknessof plate (b) is between 3 and 20 mm, preferably between 5 and 15 mm andmore preferably between 5 and 10 mm.
 11. Transparent plate laminateaccording to one or more of claims 8 to 10, characterized in that thethickness of plate (c) is between 3 and 15 mm, preferably between 5 and15 mm and more preferably between 8 and 13 mm.
 12. Transparent plateaccording to one or more of claims 8 to 11, characterized in that thethickness of the plate laminate is in the range between 30 and 100 mmand preferably in the range between 40 and 80 mm.
 13. Transparent platelaminate according to one or more of claims 8 to 12, characterized inthat plates (a)-(a), (a)-(b) and (b)-(b), independently of each other,are connected with a bonding agent selected from the group consisting ofcasting resins or reactive resins based on polyurethanes,polyvinylbutyral (PVB), cross-linked polyurethanes, partly cross-linkedpolyurethanes, polyureas, epoxides, unsaturated or saturated polyesters,polybutylene terephthalates (PBT), poly(meth)acrylates, silicones,silicone resin polymers, or from the group of hot-melt adhesives,coatings or sealants selected from the group consisting of hot-meltadhesives based on polyethylene or copolymers thereof, particularlyethylene vinylacetate (EVA), or polyvinyl acetate or mixtures thereof.14. Transparent plate laminate according to one or more of claims 8 to12, characterized in that plates (a)-(c), (b)-(c) and (c)-(c),independently of each other, are connected with a bonding agent selectedfrom the group consisting of casting resins or reactive resins based onpolyurethanes, polyvinylbutyral (PVB), cross-linked polyurethanes,partly cross-linked polyurethanes, polyureas, epoxides, unsaturated orsaturated polyesters, polybutylene terephthalates (PBT),poly(meth)acrylates, silicones, silicone resin polymers, or from thegroup of hot-melt adhesives, coatings or sealants selected from thegroup consisting of hot-melt adhesives based on polyethylene orcopolymers thereof, particularly ethylene vinylacetate (EVA), orpolyvinyl acetate or mixtures thereof.
 15. Transparent plate laminateaccording to one or more of claims 8 to 14, characterized in that aplate (a) is oriented outward or is farthest away from the object to beprotected.
 16. Transparent plate laminate according to one or more ofclaims 8 to 15, characterized in that a plate (c) is oriented inward oris closest to the object to be protected.
 17. Transparent plate laminateaccording to one or more of claims 8 to 16, characterized in that theweight per unit area of the plate laminate is in the range between 50and 150 kg/m², preferably in the range between 50 and 120 kg/m² and mostpreferably in the range between 50 and 100 kg/m² and meets therequirements of the standard NATO AEP-55 STANAG 4569, level II or Ill.18. Use of a transparent plate laminate according to one or more ofclaims 8 to 17 as part of an armored glass or as part of a bullet-proofvest.
 19. Use of a transparent plate laminate according to one or moreof claims 1 to 7 as fire prevention glazing, fireplace sight glass,ceramic hob, substrate for semiconductor materials or substrate formagnetic storage plates.